Crude oil from the Deepwater Horizon oil spill in the Gulf of Mexico causes severe defects in the developing hearts of bluefin and yellowfin tuna, according to a new study by a team of NOAA and academic scientists.

Atlantic bluefin tuna, yellowfin tuna, and other large predatory fish spawn in the northern Gulf during the spring and summer months. In 2010, their spawning season directly coincided with the Deepwater Horizon spill. Their embryos, which float near the ocean surface, were potentially in harm’s way as crude oil rose from the damaged wellhead to form large surface slicks.

“We know from the 1989 Exxon Valdez spill in Prince William Sound that recently spawned fish are especially vulnerable to crude oil toxicity,” said Nat Scholz, Ph.D., leader of the ecotoxicology program at NOAA's Northwest Fisheries Science Center in Seattle. “That spill taught us to pay close attention to the formation and function of the heart.”

What Did We Find?

Crude oil contains mixtures of polycyclic aromatic hydrocarbons, or PAHs. These PAHs adversely affect heart development in the two species of tuna, and an amberjack species, by slowing the heartbeat or causing an uncoordinated rhythm, which can ultimately lead to heart failure. According to the study, the thresholds for developmental defects are very low, in the range of approximately 1-15 parts per billion—within the PAH concentration range of water samples collected during the spill.

Severely affected fish with heart failure and deformed jaws are likely to have died soon after hatching.

The NOAA team has shown in previous work that fish that survive transient crude oil exposures can suffer subtle and transient changes in heart rhythm during development. These changes can permanently impair cardiac function and swimming performance at later life stages.

“This creates a potential for delayed mortality,” said Dr. John Incardona, NOAA research toxicologist and the study’s lead author. “Swimming is everything for these species.”

How Did We Do It?

A major difficulty facing the researchers was access to live animals. Due to their fragile nature, it was next to impossible to assess the effects of Deepwater Horizon crude oil on live embryos and larvae collected in the vicinity of the spill. Also, there are only a few facilities in the world capable of spawning adult tunas in captivity.

Instead, the team brought the oil to the fish. Samples of crude oil were collected from the field and transported to land-based hatcheries in Australia and Panama with spawning broodstocks of bluefin and yellowfin tuna. This provided access to embryos and larvae, allowing the scientists to emulate environmentally realistic crude oil exposures for both species.

Learning from the Exxon Valdez Oil Spill

This study is a culmination of more than two decades of NOAA research on crude oil toxicity to fish early life stages.

In the aftermath of the Exxon Valdez spill 25 years ago, NOAA discovered that PAHs from crude oil are highly toxic to the embryos and larvae of Pacific herring and pink salmon that spawned in oiled nearshore habitats. In more recent years, investigators at NOAA’s Northwest Fisheries Science Center have shown that these developmental deformities are primarily caused by heart defects.

The effects of Deepwater Horizon crude oil on tunas were very similar to effects previously observed in other fish species exposed to oil. Given this consistency, the authors suggest there may have been cardiac-related impacts on swordfish, marlin, mackerel, and other fish in the northern Gulf. “If they spawned in proximity to oil, we’d expect these types of effects,” said Incardona.